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The Wonder Book of Chemistry by  Jean Henri Fabre





T last they were about to see this famous gas, oxygen, this king of elements that in the past few days had again and again been referred to by Uncle Paul in his talks, but always wrapped in the mystery that enshrouds the unknown. Now it was to be released from its imprisonment in the chlorate of potash and subjected to tests that promised to be interesting. Emile even dreamed about it in the night, so much had his mind dwelt on this gas that makes things burn. In his dreams he saw the glass balloon and the bent tube cutting up all sorts of silly capers on the circular edge of the brazier, while the irascible chlorate and its comrade, the dioxid, looked on curiously through the glass walls of their prison. In the presence of the real things about which he had dreamed the night before—confronting, that is, the various preparations for the coming experiment—the boy was moved to fresh laughter when his uncle put the balloon in place over the live coals.

They had not long to wait. Before it seemed possible that the heat could have had time to act, and with no visible change in the contents of the balloon, the water in the bowl began to bubble at [188] the end of the tube, a sure sign that gas was being set free. The support already made from the bottom of a flower-pot was arranged in position in the bowl of water, and a large bottle of two or three liters' capacity and with a wide mouth was filled brimming full of water, stopped with the palm of the hand, and turned upside down on the support. Emile steadied it with his hand. The gas passed through the hole in the middle of the support, and rose through the water in a tumultuous and uninterrupted series of big bubbles, so that in a few moments the bottle was filled with the gas that had displaced the water. Then Uncle Paul took a tumbler, plunged it into the bowl, and in this glass of water set the mouth of the bottle, of course without letting it come out into the air and lose any of the imprisoned oxygen. That done, the bottle, standing upside down in the glass of water, which served to exclude the air from outside, was set safely away in a corner of the room to wait until it was needed for some experiment still to come. A second bottle took its place in the bowl, was filled with gas in the same way, and set aside for future use, after which a third and then a fourth went through the same operation. The supply of oxygen in the balloon seemed inexhaustible.


"There seems to be a lot of oxygen in a handful [189] of chlorate," remarked Emile, surprised at the quantity of gas being released.

"Yes, it is no small amount, as you see, for our four bottles hold almost a dozen liters, taken together."

"And those dozen liters of oxygen were all in that little heap of chlorate?"

"All were in the small amount of chlorate used. Was n't I right in calling this salt a rich storehouse? The chlorate is more than abundantly fed with oxygen; it is crammed to the limit. The gas taken captive by chemical combination is compressed in great quantities into very small bulk. But all is not over yet; I hope to fill this bottle, too."

Therewith Uncle Paul placed on the support in the bowl an odd-looking, elongated bottle that had hardly any neck, being of nearly the same width at top and bottom. He did not say where it came from, but his nephews thought they recognized it as an old pickle-jar. They were smiling at seeing this homely utensil pressed into such serious service, when their uncle resumed his talk.

"Does my tall jar make you laugh? Because it once held pickles, do you think it unworthy now to hold oxygen? Away with such false pride, my lads! Let us use the things at hand and not sigh for expensive luxuries. But for this once we will carry out our program in a worthy manner, just as we should if we had a better-equipped laboratory at our disposal.

"Here is something that chemists call a gage. It is a tall glass cylinder with a base to stand on. [190] I proceed to fill it with oxygen while there is still some left in the balloon, and we see that the gas enters slowly, the supply being nearly exhausted. In the balloon, the contents of which show no change in appearance, there is left the dioxid of manganese just as it was when I put it there. It has suffered neither loss not gain, but it has promoted the decomposition of the chlorate by enabling the heat to act upon it evenly. The balloon itself, too, remains uninjured and ready for further use when needed. As to the chlorate, it has now lost all its oxygen, and is thus changed to that white substance we saw yesterday remaining on the embers after we had thrown on a pinch of chlorate and watched it quicken the fire. In short, it has turned into chlorid of potassium. So much for that. Now let us put our supply of oxygen to some use, beginning with that in the gage."

Carried from the bowl to the table with the usual precautions—that is, being first closed with the palm of the hand while still standing upside down in the water—the gage was set upright on its base and covered with a piece of glass pending certain preparations consisting of fastening a short piece of candle to an iron wire, as had been done before in the experiment with nitrogen. Uncle Paul lighted the candle, allowed the flame to get bright and full, and then blew it out; but the wick was still glowing from the combustion that had been so suddenly arrested.

"The candle which I have just blown out," said he, "but which still shows a rod glow at the end [191] of the wick, I am going to lower into the oxygen in the gage. What will happen? We shall see."

Removing the piece of glass, he suited the action to the word. Piff !  A slight explosion was heard and the candle, relighting itself unaided, burned with radiant brilliance. It was extinguished once more, the wick still retaining its glowing spark, and lowered again into the gage of oxygen. Another piff!  and the flame reappeared, burning with remarkable brightness. Again and again, with care to leave the wick slightly aglow, the candle, extinguished with a breath, quickly relighted itself when it was lowered into the oxygen. Each time a little explosion preceded the revival of the flame. Emile clapped his hands with delight at this repeated renewal of the flame, always so prompt and so complete.

"How different it was with nitrogen, the partner of oxygen in the air we breathe," he observed. "Oxygen relights all of a sudden what is just about to stop burning altogether, but nitrogen puts out what is already well on fire. Could n't I try my hand at this fine experiment, Uncle?"

"Surely; why not? But I must inform you that the oxygen in the gage is by this time nearly exhausted, the candle having used up a little every time it was relighted."

"But there's plenty more in those four bottles."

"I am keeping that for other experiments of still greater importance."

"What shall I do, then?"

"You must content yourself with my old pickle- [192] jar that I took the pains to fill with oxygen, counting on it to take the place of a regular gage."

"I'll do that and be glad to."

"A wise decision, for the old pickle-jar will render you yeoman service. My chief reason for using it is to show you, by adding example to example, that instructive experiments are possible even with the commonest utensils. Our gage here is a luxury, an unheard-of extravagance in our little village. Almost any kind of bottle, any caper- or pickle-jar, provided only it has a wide mouth to admit the candle, would serve very well in performing the striking experiment you now wish to repeat. All right, then; you shall repeat it."

The jar being placed on the table, Emile began the lighting and relighting of the candle, blowing it out and rekindling the flame again and again. The experiment had not gone better even with the regular gage.

"There, now," said his uncle, "does n't my pickle-jar answer the purpose admirably?"

"Yes, splendidly."

"It is the contents and not the container that we should give our attention to. If we supply it with oxygen the candle will relight itself, no matter what the oxygen is in, whether the chemist's gage or the paltry pickle-bottle. As a close to the experiment, leave the candle in the jar to burn as long as it can. You will see how fast it will be used up."

And, indeed, the candle, immersed in oxygen, did not fail to burn with devouring rapidity. It showed no longer the calm flame that is maintained in ordi- [193] nary air, but a furious tongue of fire, extraordinarily bright and excessively hot, making the wax melt and run down in big drops. The substance of the candle was literally devoured rather than burned, and it was evident that in a few minutes there would be consumed in this energetic gas enough wax to last an hour in atmospheric air. Finally the flame died down for lack of oxygen, and Uncle Paul resumed his talk.

"Before continuing these spectacular experiments with oxygen," said he, "let us quiet our emotions with a little interlude. You know the characteristics that enable us to recognize an acid,—first the sour taste, and then the property of turning blue flowers red. But it is not always practicable to test an acid by the sense of taste, the flavor sometimes being very weak or even quite imperceptible. The test with blue flowers is a better one. But, unfortunately, violets and other blue flowers redden with some difficulty when the acid is weak. Chemistry has found the blue coloring matter of lichens more easily affected. You know those peculiar growths that look like flaky crusts on the bark of trees and even on the surface of the hardest rocks. They are of vegetable origin, and are called lichens. One species found on rocks near the sea furnishes a blue substance called litmus. The druggist has this for sale in the form of little cubes, ashy-blue in color and known as litmus tablets. If you dissolve one of these in a little water, you obtain a pale violet-blue liquid called tincture of litmus.

"This tincture is one of the most convenient [194] tests for applying to acids, as it reddens much more easily than do blue flowers. To illustrate, I pour into this glass two fingers of the litmus tincture; then I dip the end of a glass tube or of a common straw into the acid in this bottle, an acid I have already referred to as derived from sulphur,—sulphuric acid, in fact. I do not dip it deeply, but merely touch it to the liquid, and with this barely moistened straw I stir the blue tincture, which immediately turns red in proof (if I did not already know it) that what my bottle contains is an acid."

"If the tincture of litmus is turned red by an acid," said Jules, "it ought to be turned green by a soluble oxide, just as violets are; and so it would help us to find out whether a thing was an oxid or not."

"It is perfectly natural to expect that, after seeing what happens to blue flowers; and yet it is not the case. Lime and other soluble oxids do not turn the blue of litmus green, but leave it unchanged. However, this lack is atoned for by another characteristic. Once reddened by an acid, litmus is turned blue again by means of a soluble oxid. Into the contents of this glass, just now changed from blue to red by the addition of sulphuric acid, I drop a tiny particle of lime, and the liquid returns to its original blue. A second time I apply a trace of the acid with the end of this straw, and a second time the tincture turns red. Lime being [195] applied in its turn once more, the blue color reappears. These changes from blue to red and from red to blue might be repeated indefinitely. Here we have, then, a perfect test for determining whether a substance is an acid or an oxid, provided only, of course, that it is soluble in water. Wherever reddens the blue tincture of litmus is an acid, and what restores its blue to the tincture previously reddened by acid is an oxid.

"If we had no litmus—and the lack of it would be no great matter—we should have to be contented with blue flowers. A bunch of violets would first be crushed and stirred in water, and the bluish liquid thus obtained would be strained and set aside to serve the purpose of litmus. But it would show a difference in one respect; though acids would turn it red, oxids would not turn it blue again, but green; and of course the blue liquid would turn green immediately when acted upon by an oxid, without having to be previously reddened by an acid. It is to be noted, further, that weak acids might not be able to change the blue of violets to red, and therefore litmus preferable as a test.

"Our interlude is over and we will go on with the performance. We are going to burn several substances in oxygen and watch their manner of burning. First comes sulphur.

"Adopting the method you saw employed when we tried in vain to make phosphorus and sulphur burn in a bottle filled with nitrogen, I make a little cup out of a bit of broken earthenware, and bend the end of an iron wire into a circle for holding this [196] cup. The iron wire is then passed through a large cork stopper that will serve to hold it in place in the bottle rather than to stop up the bottle itself. Hence it does not matter much if the cork is too large for the bottle. A small disk of stout cardboard laid over the mouth of the bottle would answer just as well. The end of the wire, projecting above the supporting cork or cardboard, will serve as a handle for lowering or raising the cup so as to bring it into or near the center of the bottle, in the midst of the supply of oxygen."

Having finished these preparations, Uncle Paul carefully took up one of the large bottles in reserve, together with the glass full of water in which it stood and which served to close its mouth. These, without disturbing their relative position, he carried to the bowl, and there, under water, the glass was removed and replaced by the palm of the hand applied to the bottle's mouth. In this way it was possible to set the bottle upright on the table without bringing its contents into communication with the outside air. A small sheet of glass laid over the mouth closed the bottle, serving as it had done before as a temporary stopper. By means of the iron wire passing through the cork stopper, the cup, which had previously been filled with small fragments of sulphur, was so adjusted that it would take its proper place in the bottle when all was ready for it. Then Uncle Paul set fire to the sulphur and lowered the cup containing it into the oxygen. Thus suspended in the middle of the bottle and held there by its cork support, the cup of [197] sulphur required no further attention on the part of the experimenter, and there was nothing more to do but watch the result.

Everyone knows how slowly and with what a dim light sulphur burns under ordinary conditions. Hence the novelty of the spectacle now presented to the astonished gaze of the two young chemists. At their uncle's bidding the shutters had been closed, so that no daylight should get in and dim the splendor of the burning sulphur. It burned with an ardor unapproached by any brimstone match that was ever made. A fantastic radiance of a beautiful violet blue, rivaling in purity the rainbow's purple stripe, emanated from this wonderful illuminant and filled the room with so strange a glow that one might have fancied oneself transported to some other world where the sun is blue.

"Magnificent, magnificent!" cried Emile, clapping his hands with enthusiasm.

The fume of burning sulphur, escaping in puffs from the bottle and almost suffocating in their intensely pungent odor, tended somewhat to spoil this fairy-like illumination, otherwise so perfect; and so Uncle Paul, as soon as the flame began to die down, had the shutters and windows opened.

"It is all over," said he; "the sulphur has used up its supply of oxygen. I will not dwell on the splendors you have just witnessed, as your eyes have done better justice to them than could any words of mine. They have told you that sulphur burns in oxygen with a heat and a brillance that it does not have when burning in ordinary air. I [198] will pass on to inquire what has become of the sulphur we have just seen burning so brightly. What has resulted from its combination with oxygen? The result is an invisible gas with a pungent odor, a gas that makes one cough,—the same gas, in fact, that comes from a lighted match. A little of it has escaped into the room—our sense of smell and our coughing tell us that—but a good deal is still left in the bottle. Let us consult our tincture of litmus and see what information it will give us. I pour a little into the bottle and shake it up, whereupon the blue color immediately turns red. What does the litmus say?"

"It says that the sulpur has turned into an acid by burning," replied Jules.

"And it's a good thing it does say so," put in Emile, "for it would n't have been very pleasant to taste it; and, besides, you can't even see it. That litmus is certainly a convenient thing to have around."

"Very convenient," assented his uncle. "Here is something that can be neither felt nor seen, and yet a very real thing that takes you by the throat and makes you cough worse than if you had the whooping-cough. We wish to know what it is, and our litmus on being consulted answers: 'It is an acid.' "

"Does it say, too, that it is sour?"

"Undoubtedly. What reddens litmus and blue flowers, is always sour."

"But how to make sure that the litmus and the [199] violets tell the truth? I can't stick my tongue down into the middle of the bottle."

"The invisible gas from the burnt sulphur will mix with water, and so a good deal of it is absorbed in the tincture of litmus that I shook up in the bottle. We know this to be so from the effect it has on the tincture, which by itself is only water colored by a tiny particle of matter that has no flavor. Let us taste this liquid reddened by the intermixture of the gas, and we shall find out how our invisible gaseous compound tastes. Wet your finger freely in it without fear of too large a dose. It takes a good deal to make any impression on the tongue."

With their uncle to set them an example the boys tasted the liquid several times to make sure they had its flavor.

"Weak vinegar," pronounced Emile, smacking his lips; "very weak vinegar."

"Weak, if you like, but still vinegar,—that is to say, acid."

"It's nothing like so strong as phosphoric acid; that eats away your very flesh."

"Our sense of taste, then, agreeing with the tincture of litmus, tells us that sulphur by combining with oxygen in burning becomes an acid. It is this invisible gas, therefore, that has that pungent odor and makes us cough; and it is called sulphurous acid."

"You told us," said Jules, "of another acid made from sulphur, sulphuric acid, which you just used [200] to turn the tincture red. Then sulphur makes two acids, does it?"

"Yes, my boy, sulphur makes two acids, one with less oxygen and one with more. The one with less oxygen, and so the weaker, the less sour, is sulphurous acid; the other, richer in oxygen and therefore stronger and sourer, is sulphuric acid. By simply burning, either in ordinary air or in pure oxygen, sulphur takes on a certain amount of oxygen and no more, being thus changed to sulphurous acid; but by roundabout methods known to chemistry it can be made to take a larger dose, and in doing so it becomes sulphuric acid. Enough about sulphur. Let us next see what will be the result of burning charcoal in oxygen."

A piece of charcoal no bigger than one's little finger was fastened to one end of an iron wire and the other end passed through a small disk of cardboard that was to rest on the mouth of the bottle of oxygen. Uncle Paul then lighted the charcoal in the flame of a candle, but only enough to make it glow at one little point, and in this condition he lowered it into a fresh bottle of oxygen, conducting the operation in the same manner as with the sulphur.

The spectacle that followed rivaled in beauty the one just applauded so enthusiastically by Emile. At the point kindled by the candle, a spark so faint as to be hardly visible, a flame burst forth,—bright, ardent, irresistible,—and, spreading rapidly through the charcoal, soon turned it into a dazzling little forge. It gave an intensely white light, [201] with little sparks snapping and darting in all directions, like so many shooting stars shut up in the bottle. It had taken but an instant to set the charcoal all aglow in a way impossible with any draft of ordinary air. Without moving his eyes from this brilliant spectacle, Emile gave utterance to his thought:

"This heat and this bright light and these sparks I can make come when I blow with the bellows on burning charcoal. At the spot just under the nozzle of the bellows the charcoal burns almost as brightly as this, here, in the bottle."

"That is quite natural," rejoined his uncle. "With the bellows you send out air,—that is to say, oxygen mixed with a good deal of nitrogen, this latter weakening the effect. But by a sufficiently rapid renewal of this air that helps things to burn, the glowing charcoal may be made to brighten up and look much like this before us in the bottle of pure oxygen."

The supply of oxygen being at last used up, the unconsumed charcoal turned dimmer and dimmer, and then quite black. The shutters, which had again been closed, were reopened to let in the daylight, the previous admission of which would have greatly lessened the effect of the spectacle.

"What has become of the charcoal that was burnt up? That is the problem we must now solve," said Uncle Paul. "There is left in the bottle an invisible gas having scarcely any odor; and if we trusted only our smell and eyesight, we might conclude that the contents of our bottle had [202] not changed in the least. But let us submit these contents to various tests more decisive than those of smell and sight, and we shall find that there has been a decided change. First, it is safe to predict that, if the charcoal which burned so brilliantly in the beginning will now no longer burn in the bottle, a lighted candle will not burn there any better. Watch. I lower this candle, well alight, into the bottle, and it is hardly inside the neck before it goes out. Consequently, there can be no more oxygen left, for if there were, the candle, as you know, would burn with a bright flame.

"Still another test: I pour into the bottle a little tincture of litmus and shake it up thoroughly so as to give the gas a chance to act on the liquid. The blue tincture changes its color to a very pale red. Shaken up with oxygen, the same tincture would not show the slightest change. Hence, we have here another acid made by burning something in oxygen. We are now convinced that the oxygen in the bottle has been converted into another gas, no less clear, no less invisible, but endowed with very different properties; and this difference, it is plain enough, can be due only to the addition of charcoal (or carbon, which is virtually the same thing) to the oxygen. Hence we must conclude that in this gas, here in the bottle,—a gas so colorless that we cannot see it,—there is at least some slight quantity of carbon, that substance known to us as hard and heavy when we see it in coal."

"I see now that it must be so," assented Emile; "but if any one had told me, without being able to [203] prove it, that there was carbon in a gas as invisible as air, I should n't have been in a hurry to believe him. What do you say, Jules?"

"I say that it is hard to get used to the idea that a thing we can't see or feel can have carbon in it. If Uncle Paul, instead of leading us step by step to where we are now, had begun by saying that there is carbon in this bottle in which I can see nothing at all, we should have looked at him in the greatest astonishment. But the proofs are there and can't be got rid of. The charcoal in burning has changed to a gas that turns the litmus tincture red, and so it must be an acid called— Uncle has n't told us yet what it is called."

"Consult your grammar of chemistry, and find the name for yourselves."

"That 's so! I'd forgotten all about that. Charcoal is the same as carbon, and the ending ic  added to carbon  makes carbonic. The gas that comes from burning charcoal is carbonic acid."

"Is it sour like the others, this carbonic acid?" asked Emile.

"Of course, but so slightly that in the gas we have here in our bottle this acid quality is barely perceptible. The litmus, instead of turning a decided red, took on only a faint winy tinge, and to the sense of taste the sourness would be correspondingly feeble. But some day an opportunity will occur to convince you that carbonic acid is indeed sour to the taste. Now let us put our third bottle of oxygen to its intended use. In it I propose to burn some iron, a thing that Emile was inclined to think impossible [204] the other day. And I shall make this iron burn without having to heat it red-hot beforehand in a forge, as the blacksmith does with his iron before hammering it into shape. I shall set it on fire with a piece of lighted tinder, as if it were a train of gunpowder."

"And the iron will catch fire just from that piece of tinder?" was Emile's wondering inquiry.

"Certainly; gunpowder could n't do it better. Here is an old watchspring, broken at one end and of no further use. It is a bit of refuse I got at the clockmaker's. But instead of this spring, which is admirably suited to the requirements of our next experiment on account of its thin flat ribbon-shape, offering plenty of surface to the action of the oxygen, we could use an iron wire about as fine as a medium-sized needle, first cleaning it well with a file or, better, with sandpaper. The watchspring, however, is to be preferred. I begin by heating it over some live coals in order to take away its stiffness and make it supple. Then I wind it around a slender rod of some sort—a penholder or a lead-pencil will do—to give it a spiral or corkscrew form. Next I take a pair of stout scissors and cut one end of our corkscrew into a point, which I stick through a bit of tinder about as large as your finger-nail. Finally, the other end of the corkscrew is passed through a small disk of cardboard which will be placed over the bottle's mouth and will hold the metal ribbon in position in the midst of the oxygen. The spiral of our corkscrew should be pulled out to such a length as to bring its lower end into the middle of the bottle. If a wire is used instead of a watchspring, the same [205] mode of operation should be followed,—the same winding of the wire into a corkscrew form, the wire being first heated if necessary and also (an indispensable detail) cleaned with sandpaper, and the same use of tinder attached to the lower end of the spiral."

All these arrangements being carefully completed, the third bottle of oxygen was set on the table. In preparing for it, Uncle Paul had taken care not to fill this bottle entirely with the gas, but to leave several inches of water in the bottom.

"There's some water in the bottle," Emile pointed out, resolved not to let any detail of this curious experiment escape him.

"Yes, and it is there for a purpose. If there were none, some would have to be poured in now. A considerable depth of water at the bottom is necessary if we wish to keep our bottle for future experiments. You will very soon see why the water is needed. Close the shutters and I will begin."

As soon as the room had been darkened, the tinder was lighted and the spiral ribbon lowered into the oxygen. The tinder flared up suddenly and burned with a bright flame. Next followed a moment of indecision; the iron was taking fire; then it was well on fire and presented the appearance of a piece of fireworks. This marvelous flame, feeding on metal, was seen to make its way upward in a spiral curve as a fire spreads from bottom to top of a winding staircase. Snappings and cracklings and sprays of sparks accompanied the process. At the end of the ribbon there collected and hung suspended a globule [206] of molten metal of dazzling brightness. Growing too heavy, it detached itself and fell. Still intensely hot it plunged through the water with a sharp hissing sound and in a state of redness reached the bottom of the bottle, which it softened as it flattened itself out there. Other globules followed, dropping one by one from the flaming spiral, and despite the cooling effect of the water the largest ones retained enough heat to melt the glass a little and sink into it.

The boys stood silent before this magic spectacle of iron devoured by oxygen; but Emile was not without his fears. The hissing when the melted globules fell, the failure of the water to extinguish immediately these drops of liquid fire, the snapping and spluttering of the burning watchspring, the showers of sparks and the cracking of glass, all united to furnish a spectacle of startling strangeness. Holding his hands before his face to protect it, the boy was evidently expecting some terrific explosion. But all ended very quietly, and only the bottle, cracked in several places, suffered any damage from this chemical celebration. Then Uncle Paul broke the silence that had ensued.

"Well, Emile, does iron burn? Are you convinced at last?"

"I shall have to be," he replied. "Iron burns, and it burns fast. It was like a little show of fireworks."

"And you, Jules,—what do you think of my experiment?"

"I think it even finer than the one with magnesium. That metal made a light such as I had never seen before; but magnesium itself was something [207] new to us, and so it could not surprise us very much to see it burn. With iron the case is different: we are used to this metal and have so often seen it resist fire that when we see it burn like wood-shavings it strikes us both as something wonderful. But what surprised me most was to see those drops of melted iron stay red-hot for some moments under water."

"Those globules that fell from the spiral as the flame ascended are not iron, but oxid of iron, formed by the combination of that metal with oxygen. I will take out of the bottle those that are not stuck to the glass. They consist, you see, of a black substance that crumbles easily in the fingers. If they were of iron alone, they would not do this. Their softness points to the presence of another element, and this element, as I said, is oxygen. You will find this same oxid of iron in the tiny scales, black and easily broken, that fly off when the blacksmith hammers red-hot iron on his anvil. Both are iron that has been through fire, iron that has become oxidized. Notice also, on the inside surface of the bottle, a light layer of fine reddish dust that was not there before. What can this red dust be? What soes it look like?"

"It looks a good deal like iron-rust," replied Jules; "at least it has exactly the color of iron-rust."

"And it is iron-rust,—that and nothing else. Remember this little fact, for it will be useful to you later: iron-rust is iron combined with oxygen."

"Then were there two oxids of iron made in the bottle?"

[208] "Yes, two, but in very unequal parts. The more abundant is the black substance; the other is the red dust deposited on the inner surface of the glass; and this latter is richer in oxygen than the other. I will not dwell on this subject further now, as I shall come back to it later. Notice, finally, the cracks in the bottom of the bottle and the globules of oxid embedded in the thick glass."

"Those drops of oxid must have been terribly hot," said Emile, "to melt the glass like that after going through water. I've often seen drops of fire fall from the fat when you singe a roast, but I never knew before that there could be ever so much hotter ones."

"Was I right, then, to leave some water in the bottle?"

"I should say so! If you hadn't, the bottom would have been bored clear through."

"More than that; the bottle would have been shattered to pieces by the sudden intense heat. The first drop to fall from the ribbon would have ended the experiment by breaking the bottle. But with this protecting layer of water our bottle has held together and, although cracked, can still be used."

There still remained a fourth bottle of oxygen. Also, safely caged and well supplied with bread crumbs, the sparrow was watching the proceedings. In the midst of plenty, captivity did not seem to depress its spirits unduly. But now its turn had come to be experimented upon, though without any fatal ending in prospect this time, Uncle Paul had assured his hearers. The death of the bird's unlucky pre- [209] decessor had shown the boys that nitrogen is unbreathable, and that in this gas in which a flame goes out life also is extinguished. What new truth was this sparrow to teach them? It was about to show them the effect of oxygen when breathed unmixed with any other gas. Their uncle took the sparrow and put it into the remaining bottle of oxygen.

At first nothing unusual occurred. Then, after a short interval, the saucy bird became even more alert, brisker in its movements, livelier in every way, than under natural conditions. Hopping about, flapping its wings, stamping with its feet, pecking the glass walls of its prison with furious beak, the little creature was evidently in a burning fever which was fast using up its strength. It panted as if its little breast would burst with the wild pulsations of its heart. Its open beak denoted extreme fatigue, and yet the feverish restlessness still increased. To prevent a sad ending to this scene, Uncle Paul hastened to put the bird back into its cage, where the fever subsided in a few minutes.

"My demonstration is finished," he announced: "oxygen is a breathable gas; an animal can live in it; which not the case with nitrogen. But life goes on more intensely in oxygen than is altogether agreeable, as we have just seen from the sparrow's extraordinary agitation."

"Never before," said Jules, "have I seen a sparrow so worked up. It acted like one possessed. Why did you take it out of the bottle so soon?"

[210] "Because it would have killed the bird to keep it in there much longer."

"Is oxygen a gas that kills?"

"On the contrary, it give life."

"Well, then, I don't see—"

"Recall the lighted candle that was lowered into oxygen. It went on burning there, but with a devouring ardor and an immoderate expenditure of wax. The flame was of superb brilliance and vigor, but of short duration. The fuel that would have kept it going a long time under ordinary conditions was used up in a few seconds. It is much the same with life; it goes at an unnatural pace in pure oxygen, uses itself up too rapidly to last long. We might express it by saying that the animal machine is geared too high, and hence, like all over-driven machines, breaks down and stops. You saw how the bird performed all sorts of mad antics, as if violently intoxicated. At that rate its poor little machine would surely have gone to pieces very soon, and that is why I took the exhausted creature out of the bottle, wishing to keep it for another and final experiment. Take good care of it until to-morrow."

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